Transceivers are used in a wide range of communications applications that require the transmission and reception of electromagnetic signals. Such transmission and reception may be accomplished over various wireless and fixed communications channels. Examples of wireless applications of transceivers include but are not limited to cellular telephones, cordless telephones and two-way radios. Examples of fixed-line applications of transceivers include but are not limited to local area networks, cable systems and optical fiber systems.
Transceivers that utilize time-division multiplexing generally require a transmit/receive (TX/RX) switch to isolate the transceiver's receiver from its transmitter during reception and to isolate its transmitter from its receiver during transmission. Transceivers generally also include electro-static discharge (ESD) circuitry to provide protection from ESD events and their associated transients. Such ESD circuitry is particularly important for the radio frequency (RF) input device of a transceiver, which is highly sensitive to ESD.
ESD protection is generally achieved by adding ESD protection elements to the input/output pad of a transceiver in order to divert transients caused by an ESD event to ground. One example of such an ESD protection element is the thyristor. Some sources define thyristors and silicon-controlled rectifiers (SCRs) as synonymous; others define SCRs as a subset of thyristors. Throughout this document, the term “thyristor” means both thyristors and SCRs.
Unfortunately, ESD protection elements tend to add parasitic resistance, capacitance and/or inductance to an RF circuit. Since RF circuits are extremely sensitive to any added parasitics, ESD protection at high frequencies is difficult to achieve and is as a result sometimes altogether omitted.